Granulated powder of low-molecular polytetrafluoro-ethylene and powder of low-molecular polytetrafluoro-ethylene and processes for producing both

ABSTRACT

This invention provides a granulated powder of low-molecular-weight polytetrafluoroethylene reduced in the possibility of the powder being blown up or adhering to hoppers, a low-molecular-weight polytetrafluoroethylene powder obtainable by suspension polymerization, and methods of producing these. The present invention provides a low-molecular-weight polytetrafluoroethylene-based granulated powder which is obtained by a granulation treatment of a low-molecular-weight polytetrafluoroethylene particle comprising a low-molecular-weight polytetrafluoroethylene having a number average molecular weight of not higher than 600000.

TECHNICAL FIELD

The invention relates to a low-molecular-weightpolytetrafluoroethylene-based granulated powder, a low-molecular-weightpowder, and methods of producing these.

BACKGROUND ART

Low-molecular-weight polytetrafluoroethylene species have so far beenused, for example, as additives to ink or cosmetic compositions or likeother base materials for the purpose of reducing friction and improvingslip characteristics on the base material surface or as additives tocoating materials for the purpose of improving the surface texture ofcoatings.

Known as low-molecular-weight polytetrafluoroethylene species are thoseobtained by emulsion polymerization (cf. e.g. Japanese Kokai PublicationS51-41085 and Japanese Kokai Publication H07-165828), those obtained bythermal degradation of high-molecular-weight polytetrafluoroethylenespecies (cf. e.g. Japanese Kokai Publication S49-39642, Japanese KokokuPublication (Patent Publication) H07-5744, Japanese Kokoku PublicationS50-15506, Japanese Kokai Publication S61-118331 and Japanese KokaiPublication S61-162503), and those obtained by subjecting moldings fromhigh-molecular-weight polytetrafluoroethylene in powder, scrap or otherform to irradiation treatment (cf. e.g. Japanese Kokoku PublicationS52-25419, Japanese Kokoku Publication S49-48671, Japanese KohyoPublication 2001-513529 and U.S. Pat. No. 3,766,031).

However, low-molecular-weight polytetrafluoroethylene powders obtainedby any of those methods have problems, namely they tend to blow up asfine dust on the occasion of addition to other materials or, when theyare electrostatically charged, they readily adhere to the hopper; thus,they are very poor in handleability.

That low-molecular-weight polytetrafluoroethylene powders are composedof very fine particles is also presumable as one of the causes of theproblems of powder blowing up and adhesion to hoppers. If the size ofthe very fine particles is increased in an attempt to solve the aboveproblems, however, there arises another problem, namely thedispersibility thereof on the occasion of admixing with other materialsbecomes poor.

There are no low-molecular-weight polytetrafluoroethylene powdersavailable in the art that have both the characteristics, goodhandleability without powders blowing up or adhering to hoppers and gooddispersibility as additives.

From the process simplification and molecular weight distributionnarrowing viewpoint, those low-molecular-weight polytetrafluoroethylenespecies which can be directly obtained by polymerization and preferablyneed no post-treatment step for molecular weight reduction arepreferred. As a method by which they can be obtained directly bypolymerization, there is known the emulsion polymerization method (cf.e.g. Japanese Kokai Publication H07-165828) but any method involvingsuspension polymerization is not known.

Further, a powder with a specific surface area of 7 to 20 m²/g is knownas a low-molecular-weight polytetrafluoroethylene species obtained bypolymerization (cf. e.g. Japanese Kokai Publication H10-147617). Thispowder, however, is disadvantageous in that its particles are readilyblown up or adhere to hoppers.

Furthermore, recent study results, among others, have revealed asuspected risk of perfluorooctanoic acid [PFOA] to the environment, andthe United States Environmental Protection Agency (EPA) announced, onApr. 14, 2003, that more intensive scientific researches concerning PFOAshould be made (cf. e.g. EPA Report “Preliminary Risk Assessment of theDevelopmental Toxicity Associated with Exposure to PerfluorooctanoicAcid and Its Salts”, Internet<URL:http://www.epa.gov/opptintr/pfoa/pfoara.pfd>).

SUMMARY OF THE INVENTION

In view of the above-discussed state of the art, it is an object of thepresent invention to provide a granulated powder of low-molecular-weightpolytetrafluoroethylene reduced in the possibility of the powder beingblown up or adhering to hoppers, a low-molecular-weightpolytetrafluoroethylene powder obtainable by suspension polymerization,and methods of producing these.

This invention provides a low-molecular-weightpolytetrafluoroethylene-based granulated powder which is obtained by agranulation treatment of a low-molecular-weight polytetrafluoroethyleneparticle comprising a low-molecular-weight polytetrafluoroethylenehaving a number average molecular weight of not higher than 600000.

This invention provides a method of producing the low-molecular-weightpolytetrafluoroethylene-based granulated powder, which comprisesproducing the above mentioned low-molecular-weightpolytetrafluoroethylene-based granulated powder from thelow-molecular-weight polytetrafluoroethylene particle by the granulationtreatment, said granulation treatment being carried out at a temperaturenot lower than 80° C. but lower than 100° C. using alow-molecular-weight polytetrafluoroethylene aqueous dispersioncomprising said low-molecular-weight polytetrafluoroethylene particledispersed in an aqueous dispersion medium in the presence of asurfactant.

This invention provides a method of producing the low-molecular-weightpolytetrafluoroethylene-based granulated powder, which comprisesproducing the above mentioned low-molecular-weightpolytetrafluoroethylene-based granulated powder from thelow-molecular-weight polytetrafluoroethylene particle by the granulationtreatment, said granulation treatment being carried out using alow-molecular-weight polytetrafluoroethylene aqueous dispersioncomprising said low-molecular-weight polytetrafluoroethylene particledispersed in an aqueous dispersion medium in the presence or absence ofa surfactant, said low-molecular-weight polytetrafluoroethylene aqueousdispersion containing a water-insoluble liquid and said surfactantamounts to not more than 5 parts by mass per 100 parts by mass of saidlow-molecular-weight polytetrafluoroethylene particle.

This invention provides a low-molecular-weight polytetrafluoroethylenepowder which has a melt viscosity not higher than 2500 Pa·s as measuredby the flow tester method at 340° C. and is obtained by suspensionpolymerization.

This invention provides a low-molecular-weight polytetrafluoroethylenepowder which has a specific surface area smaller than 7 m²/g and a meltviscosity not higher than 2500 Pa·s as measured by the flow testermethod at 340° C.

This invention provides a method of producing the low-molecular-weightpolytetrafluoroethylene powder, which comprises producing the abovementioned low-molecular-weight polytetrafluoroethylene powder bysuspension polymerization using a chain transfer agent, said chaintransfer agent being hydrogen, a lower saturated hydrocarbon or a loweralcohol, said suspension polymerization being carried out at a liquidtemperature not lower than 40° C. but lower than 100° C. using apolymerization initiator and said polymerization initiator comprises apersulfate salt or a sulfite salt and an organic peroxide.

This invention provides a method of producing the low-molecular-weightpolytetrafluoroethylene powder, which comprises producing the abovementioned low-molecular-weight polytetrafluoroethylene powder bysuspension polymerization using a chain transfer agent, said chaintransfer agent being hydrogen, a lower saturated hydrocarbon or a loweralcohol, said suspension polymerization being carried out at a liquidtemperature of 5 to 40° C. using a polymerization initiator and saidpolymerization initiator comprising not only a persulfate salt or asulfite salt and/or an organic peroxide but also a redox catalyst.

This invention provides a low-molecular-weightpolytetrafluoroethylene-based gelation product powder which is obtainedthrough heat treatment comprising heating the above mentionedlow-molecular-weight polytetrafluoroethylene powder at not lower than250° C. but lower than 340° C.

DETAILED DISCLOSURE OF THE INVENTION

In the following, the invention is described in detail.

The low-molecular-weight polytetrafluoroethylene-based granulated powderaccording to the invention (hereinafter referred to as“low-molecular-weight PTFE-based granulated powder”) is obtained bygranulation treatment of a low-molecular-weight PTFE particle comprisinga low-molecular-weight polytetrafluoroethylene species (hereinafterreferred to as “low-molecular-weight PTFE”).

The above-mentioned low-molecular-weight PTFE has a number averagemolecular weight of not higher than 600,000. When it is higher than600,000, fibrillation characteristics manifest themselves, leading toready aggregation and poor fine dispersibility in certain instances. Asfor the lower limit to the number average molecular weight of thelow-molecular-weight PTFE, a preferred lower limit may be set at 10,000,for instance, provided that the number average molecular weight iswithin the range mentioned above. When it is lower than 10,000, thevolatility at elevated temperatures becomes high, sometimes making thePTFE unsuited for use in heat-resistant coating materials, for examplecoating materials requiring baking.

The number average molecular weight of the low-molecular-weight PTFE isthe value calculated from the melt viscosity obtained by measurementusing the flow tester method.

The low-molecular-weight PTFE may be one obtained by the method ofpolymerization of low-molecular-weight PTFEs, which is to be describedlater herein, one obtained by thermal degradation ofhigh-molecular-weight polytetrafluoroethylene, or one obtained bysubjecting high-molecular-weight polytetrafluoroethylene to irradiationtreatment, provided that it has a number average molecular weight of nothigher than 600,000.

The low-molecular-weight PTFE comprises a tetrafluoroethylenehomopolymer [TFE homopolymer] and/or a modified polytetrafluoroethylene[modified PTFE].

The “TFE homopolymer and/or modified PTFE” so referred to hereinincludes, within the meaning thereof, one comprising a TFE homopolymerand containing no modified PTFE, one comprising a modified PTFE andcontaining no TFE homopolymer, and one comprising a TFE homopolymer anda modified PTFE.

The portion “polytetrafluoroethylene (PTFE)” in the above term“low-molecular-weight PTFE” generally and often means theabove-mentioned TFE homopolymer. In the present specification, however,as is evident from the above definition “the low-molecular-weight PTFEcomprises a TFE homopolymer and/or a modified PTFE”, that portion is notused without any intention of restricting the scope of the term to TFEhomopolymers but is nothing but a part of the term “low-molecular-weightPTFE” as one of the terms used herein. Thus, the term“low-molecular-weight PTFE” as a whole represents a TFE homopolymerand/or a modified PTFE.

The TFE homopolymer is obtained by polymerizing tetrafluoroethylene[TFE] alone as a monomer.

The modified PTFE mentioned above means a polymer obtained from TFE anda modifier.

The modifier in the above-mentioned modified PTFE is not particularlyrestricted but may be any of those copolymerizable with TFE, including,for example, perfluoroolefins such as hexafluoropropene [HFP];chlorofluoroolefins such as chlorotrifluoroethylene [CTFE];hydrogen-containing fluoroolefins such as trifluoroethylene; andperfluoro vinyl ethers.

The perfluoro vinyl ethers are not particularly restricted but may be,for example, perfluoro unsaturated compounds represented by the generalformula (I):CF₂═CF—ORf   (I)wherein Rf represents a perfluoro organic group. The term “perfluoroorganic group” as used herein means an organic group resulting fromsubstitution of fluorine atoms for all the hydrogen atoms bound to thecarbon atom or atoms contained therein. The perfluoro organic group maycontain one or more ether oxygen atoms.

As the perfluoro vinyl ethers, there may be mentioned, for example,perfluoro(alkyl vinyl ether) [PAVE] species in which Rf in the generalformula (I) represents a perfluoroalkyl group containing 1 to 10 carbonatoms. Preferably, the perfluoroalkyl group contains 1 to 5 carbonatoms.

As the perfluoroalkyl group in the PAVEs, there may be mentioned, forexample, perfluoromethyl, perfluoroethyl, perfluoropropyl,perfluorobutyl, perfluoropentyl and perfluorohexyl. Perfluoropropyl ispreferred, however.

The perfluoro vinyl ethers also include, among others,perfluoro(alkoxyalkyl vinyl ether) or perfluoro(alkylpolyoxyalkylenevinyl ether) species in which Rf in the general formula (1) represents aperfluoro(alkoxyalkyl) group containing 4 to 9 carbon atoms, an organicgroup represented by the formula

(in which m represents an integer of 1 to 4) or an organic grouprepresented by the formula

(in which n represents an integer of 1 to 4).

Perfluoro vinyl ethers and chlorotrifluoroethylene are preferred as themodifier in the modified PTFE, and PAVEs are preferred as the perfluorovinyl ethers.

In the modified PTFE in which such a perfluorovinyl ether as mentionedabove, for instance, is used as the modifier, the proportion (% by mass)of the modifier to the sum of the modifier and TFE is, in ordinarycases, preferably not higher than 1% by mass, more preferably 0.001 to1% by mass.

The modified PTFE may comprise one single species or two or more speciesdiffering, for example, in number average molecular weight and/orcopolymer composition, and the TFE homopolymer may comprise one singlespecies or two or more species differing in number average molecularweight, for instance.

The method of polymerization of the low-molecular-weight PTFE is notparticularly restricted but includes emulsion polymerization andsuspension polymerization, among others.

In the polymerization of the low-molecular-weight PTFE, a chain transferagent may be used. The use of such chain transfer agent makes itpossible to adjust the molecular weight of the productlow-molecular-weight PTFE and improve the dispersibility as an additiveto other materials.

The chain transfer agent is not particularly restricted but may behydrogen, a lower saturated hydrocarbon or a lower alcohol. The lowersaturated hydrocarbon includes, among others, straight or cyclic alkanescontaining 1 to 6 carbon atoms, such as methane, ethane, propane,butane, hexane and cyclohexane, and the lower alcohol includes, amongothers, alcohols containing 1 to 3 carbon atoms, such as methanol andethanol.

On the occasion of the polymerization of the low-molecular-weight PTFE,an unstable terminal group derived from the chemical structure of thepolymerization initiator to be described later herein or theabove-mentioned chain transfer agent is formed at one or each molecularchain terminus of the low-molecular-weight PTFE. The unstable terminalgroup is not particularly restricted but includes, for example, —CH₂OH,—COOH and —COOCH₃.

The low-molecular-weight PTFE may be one resulting from stabilization ofthe unstable terminal group. The method of stabilizing the unstableterminal group is not particularly restricted but may comprises, forexample, converting the terminus to a trifluoromethyl group [—CF₃] byexposure to a fluorine-containing gas.

The low-molecular-weight PTFE may also be one resulting from terminalamidation. The method of such terminal amidation is not particularlyrestricted but may comprise, for example, bringing the fluorocarbonylgroup [—COF] obtained in the above manner by exposure to afluorine-containing gas into contact with gaseous ammonia, as disclosedin Japanese Kokai Publication H04-20507.

When the low-molecular-weight PTFE is one resulting from theabove-mentioned unstable terminal group stabilization or terminalamidation, the low-molecular-weight PTFE-based granulated powderobtained in accordance with the invention or the low-molecular-weightPTFE powder of the invention, which is to be described later herein, canbe readily compatible with other materials, such as ink, coatingmaterials or cosmetic compositions, hence can show improveddispersibility therein in cases where it is used as an additive to theother materials.

The low-molecular-weight PTFE particles comprising low-molecular-weightPTFE (hereinafter referred to as “low-molecular-weight PTFE particles”)refers herein to particles not yet undergone granulation treatment, asis evident from the fact that the low-molecular-weight PTFE-basedgranulated powder of the invention is obtained from low-molecular-weightPTFE particles by granulation treatment, as described hereinabove.

The term “low-molecular-weight PTFE particles” as used hereinconceptually includes not only those particles in a solid powder butalso those particles dispersed, as a dispersoid, in the dispersionobtained by emulsion polymerization, for instance.

The low-molecular-weight polytetrafluoroethylene powder comprising thelow-molecular-weight PTFE particles (hereinafter referred to as“low-molecular-weight PTFE powder”) is a mass of low-molecular-weightPTFE particles not yet undergone any granulation treatment, as mentionedabove and, when the low-molecular-weight PTFE particles are theparticles in a solid powder, it is the solid powder itself and, when thelow-molecular-weight PTFE particles are the particles dispersed in adispersion, it is that mass of low-molecular-weight PTFE particles whichcorresponds to the powder obtained by separating the particles from thedispersion in the conventional manner, for example by flocculation,followed by drying.

The low-molecular-weight PTFE powder preferably has an average graindiameter of 0.5 to 20 μm.

The average grain diameter of the low-molecular-weight PTFE powder as soreferred to herein is the value obtained by measuring the grain sizedistribution of the powder by the laser diffraction grain sizedistribution measurement method and making a calculation assuming thatthe average grain size is equal to that grain diameter corresponding tothe 50% level of the thus-obtained cumulative grain size distribution.

The low-molecular-weight PTFE-based granulated powder of the inventionis obtained from the low-molecular-weight PTFE particles mentioned aboveby granulation treatment.

It is known in the art that polytetrafluoroethylene species having anumber average molecular weight of not higher than 600,000 can hardlyfibrillate and, according to a way of thinking, such polymers difficultto fibrillate are difficult to granulate. Thus, no granulation productsfrom low-molecular-weight PTFE particles are known in the art. Thepresent invention has now realized the granulation oflow-molecular-weight PTFE particles.

The granulation treatment comprises the step of mixinglow-molecular-weight PTFE particles with a filler, surfactant, and/orgranulation medium as desired, which is to be described later herein,the step of separating the granulation product obtained from thegranulation medium (when used), and the step of drying, if desired. Thegranulation medium is a medium in the presence of which thelow-molecular-weight PTFE particles are subjected to granulationtreatment. The granulation medium generally comprises water and/or anorganic liquid and is to be selected according to the granulation methodemployed. The granulation method is not particularly restricted butincludes, among others, the underwater granulation, warm or hot watergranulation, emulsification/dispersion granulation, emulsification/hotwater granulation, solventless granulation and dry solvent granulationmethods. It is also possible to employ the granulation methods used inthe process for producing low-molecular-weight granulated powdersaccording to the invention, which is to be described later herein, asthe underwater granulation, emulsification/hot water granulation andemulsification/dispersion granulation methods.

The underwater granulation method is a method comprising dry mixing thelow-molecular-weight PTFE particles with a filler (mentioned laterherein), if desired,: adding water, further adding a water-insolubleliquid to the water, stirring the mixture to produce liquid drops andcause the low-molecular-weight PTFE particles to be included in theliquid drops. In the liquid drops, the water-insoluble liquid presumablyexists among low-molecular-weight PTFE particle-constituting polymerchains and, when heated, this water-insoluble liquid volatilizes earlierthan water, with the result that the polymer chains are aggregated andgranulated to form a low-molecular-weight PTFE-based granulated powder.The underwater granulation method does not use any surfactant.Employable as the water-insoluble liquid are the same ones as thosewhich can be used in carrying out the method of producinglow-molecular-weight PTFE-based granulated powders according to theinvention, which method is to be described later herein.

The warm or hot water granulation method is a method according to whichthe above underwater granulation method is carried out in a conditionsuch that the temperature of the aqueous dispersion medium is raised to30 to 100° C., with or without a water-insoluble liquid contained in themixture. The amount of the water-insoluble liquid should be not largerthan 5% by mass relative to the low-molecular-weight PTFE aqueousdispersion.

The emulsification/dispersion granulation method is a method that canalso be said to be a method according to which the above-mentionedunderwater granulation method is carried out in the presence of asurfactant, namely using a low-molecular-weight PTFE aqueous dispersionresulting from dispersion of low-molecular-weight PTFE particles in anaqueous dispersion medium supplemented with the above-mentionedwater-insoluble liquid in the presence of a surfactant.

The emulsification/hot water granulation method according to which thelow-molecular-weight PTFE aqueous dispersion resulting from dispersionof low-molecular-weight PTFE particles in an aqueous dispersion medium,with or without addition of the water-insoluble liquid, in the presenceof a surfactant is subjected to granulation treatment in a conditionsuch that the aqueous dispersion is heated to a temperature not lowerthan 80° C. but lower than 100° C.

The grains obtained by the underwater granulation method, the grainsobtained by the emulsification/dispersion granulation method and thegrains obtained by the emulsification/hot water granulation method arerespectively higher in apparent density than the startinglow-molecular-weight PTFE particles and, therefore, the powders can beinhibited from being blown up.

The solventless granulation method is a method according to which anaqueous solution of a surfactant is used in lieu of the water and/ororganic solvent and this aqueous surfactant solution is admixed withlow-molecular-weight PTFE particles.

The dry solvent granulation method is a method using an organic solventas the granulation medium, without using any surfactant.

The low-molecular-weight PTFE-based granulated powder of the inventionas obtained by the above-mentioned granulation treatment preferably hasan average grain diameter of 1 to 1500 μm.

Further, the low-molecular-weight PTFE-based granulated powder of theinvention is superior in powder flowability as compared with theconventional low-molecular-weight PTFE-based granulated powders, and thepowder can be inhibited from adhering to the hopper wall surface and thehandleability thereof can be improved.

The low-molecular-weight PTFE-based granulated powder of the inventionpreferably has an average grain diameter 1 to 400 times the averageparticle diameter of the low-molecular-weight PTFE powder comprisinglow-molecular-weight PTFE particles and an apparent density 1.15 to 4times the apparent density of the low-molecular-weight PTFE powdercomprising low-molecular-weight PTFE particles (hereinafter suchgranulated powder is referred to as “low-molecular-weight PTFE-basedgranulated powder (P)”).

The average grain diameter of the low-molecular-weight PTFE-basedgranulated powder (P) is at a ratio within the range mentioned above, sothat the powder (P) is hardly blown up.

More preferably, the low-molecular-weight PTFE-based granulated powder(P) has an apparent density at least 1.2 times but at most 3 times theapparent density of the low-molecular-weight PTFE powder. Thelow-molecular-weight PTFE-based granulated powder of the invention cansatisfy both the requirement that the apparent density should be 1.15 to4 times and the requirement that the average grain diameter should be 1to 400 times.

The low-molecular-weight PTFE-based granulated powder (P) consists ofsticky grains and can be inhibited from being blown up or adhering tohoppers and, in addition, is less bulky in transportation and storage,hence the hopper and/or storage tank can be designed to be small. Theapparent density so referred to herein is the value obtained by making ameasurement according to JIS K 6891.

The low-molecular-weight PTFE-based granulated powder (P) of theinvention can be obtained with ease by the method (1) of producinglow-molecular-weight PTFE-based granulated powders according to theinvention or the method (2) of producing low-molecular-weight PTFE-basedgranulated powder according to the invention, which is to be describedlater herein.

In another aspect, the low-molecular-weight PTFE-based granulated powderof the invention preferably has an average grain diameter 1 to 3 timesthe average particle diameter of the low-molecular-weight PTFE powdercomprising low-molecular-weight PTFE particles and an angle of repose atleast 1.1 times the angle of repose of the low-molecular-weight PTFEpowder comprising low-molecular-weight PTFE particles (such granulatedpowder is hereinafter referred to as “low-molecular-weight PTFE-basedgranulated powder (Q)”).

The low-molecular-weight PTFE-based granulated powder (Q), which showsan angle of repose ratio within the above range, consists of stickygrains, so that the powder can be prevented from being blown up and thehandling thereof in such an operation as charging into a hopper can befacilitated. The angle of repose ratio may be 1.1 to 1.5 times, forinstance.

The conventional molding powder granulation products obtained bygranulation of molding powders of polytetrafluoroethylene species havinga number average molecular weight exceeding 600,000 have an averagegrain diameter exceeding 3 times, as mentioned hereinabove. On thecontrary, the low-molecular-weight PTFE-based granulated powder of theinvention can satisfy both the requirement that the angle of reposeshould be at least 1.1 times that of the low-molecular-weight PTFEpowder and the requirement that the average grain diameter should be 1to 3 times that of the low-molecular-weight PTFE powder.

Further, the low-molecular-weight PTFE-based granulated powder (Q) ofthe invention preferably has an apparent density ratio of 1.15 to 4times, like the low-molecular-weight PTFE-based granulated powder (P),in addition to the above-mentioned average grain diameter ratio andangle of repose ratio, as compared with the low-molecular-weight PTFEpowder.

The low-molecular-weight PTFE-based granulated powder (Q) of theinvention can be obtained with ease by the method (1) of producinglow-molecular-weight PTFE-based granulated powders according to theinvention, which method is to be described later herein.

In a further aspect, the low-molecular-weight PTFE-based granulatedpowder of the invention preferably has an average grain diameter 10 to4.00 times the average particle diameter of the low-molecular-weightPTFE powder comprising low-molecular-weight PTFE particles and an angleof repose not greater than the angle of repose of thelow-molecular-weight PTFE powder comprising low-molecular-weight PTFEparticles (such granulated powder is hereinafter referred to as“low-molecular-weight PTFE-based granulated powder (R)”).

The low-molecular-weight PTFE-based granulated powder (R), which iswithin the above respective ranges, is hardly blown up and, in addition,is excellent in handleability.

Further, the low-molecular-weight PTFE-based granulated powder (R) ofthe invention preferably has an apparent density ratio of 1.15 to 4times, like the low-molecular-weight PTFE-based granulated powder (P),in addition to the above-mentioned average grain diameter ratio andangle of repose ratio, as compared with the low-molecular-weight PTFEpowder.

The low-molecular-weight PTFE-based granulated powder (R) of theinvention can be obtained with ease by the method (2) of producinglow-molecular-weight PTFE-based granulated powders according to theinvention, which method is to be described later herein.

The “low-molecular-weight PTFE-based granulated powder”, when simply soreferred to herein without adding (P), (Q) or (R), includes, within themeaning thereof, the whole of the low-molecular-weight PTFE-basedgranulated powders including the above-mentioned low-molecular-weightPTFE-based granulated powder (P), low-molecular-weight PTFE-basedgranulated powder (Q) and low-molecular-weight PTFE-based granulatedpowder (R) and, further, those low-molecular-weight PTFE-basedgranulated powders which do not fall under any of the above-mentionedcategories, namely low-molecular-weight PTFE-based granulated powder(P), low-molecular-weight PTFE-based granulated powder (Q) andlow-molecular-weight PTFE-based granulated powder (R).

In the low-molecular-weight PTFE-based granulated powder of theinvention, as for the low-molecular-weight PTFE powder comprisinglow-molecular-weight PTFE particles, the low-molecular-weight PTFEpowder may be one having a specific surface area smaller than 7 m²/g anda melt viscosity of not higher than 2500 Pa·s as measured by the flowtester method at 340° C. The melt viscosity may be lower than 1000 Pa·s.That low-molecular-weight PTFE powder which has a specific surface areaand a melt viscosity within the respective ranges mentioned above ishereinafter referred to as “low-molecular-weight PTFE powder (A)”.

The low-molecular-weight PTFE powder (A) has a relatively small specificsurface area, so that the blowing up of the powder and the adhesionthereof to hoppers can be reduced. A preferred upper limit to thespecific surface area of the low-molecular-weight PTFE powder (A) is 6m²/g, a more preferred upper limit thereto is 5 m²/g, and a preferredlower limit is 1 m²/g and a more preferred lower limit is 2 m²/g.

The specific surface area so referred to herein refers to the valueobtained by making a measurement according to the BET method using asurface analyzer.

Since the melt viscosity at 340° C. of the low-molecular-weight PTFEpowder (A) is within the above range, the number average molecularweight of the low-molecular-weight PTFE is approximately 100000 orbelow. The number average molecular weight may be not higher than 40000,for instance.

The melt viscosity so referred to herein refers to the value obtained bymaking a measurement by the flow tester method at 340° C. according toASTM D 1238.

The low-molecular-weight PTFE powder (A) is preferably one having a meltviscosity within the above range and a specific surface area within theabove range and obtained by suspension polymerization.

The low-molecular-weight PTFE-based granulated powder (P) of theinvention, the low-molecular-weight PTFE-based granulated powder (Q) ofthe invention and the low-molecular-weight PTFE-based granulated powder(R) of the invention, each described hereinabove, can also be obtainedby using the low-molecular-weight PTFE powder (A).

The “low-molecular-weight PTFE powder”, when simply so referred toherein without adding (A) or (B) (which is to be mentioned laterherein), includes, within the meaning thereof, the whole of thelow-molecular-weight PTFE powders among which the above-mentionedlow-molecular-weight PTFE powder (A) and the low-molecular-weight PTFEpowder (B) to be described later herein are included, without making anydistinction among the low-molecular-weight PTFE powder (A) andlow-molecular-weight PTFE powder (B) and low-molecular-weight PTFEpowders other than these.

The method (1) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention comprises producing theabove-mentioned low-molecular-weight PTFE-based granulated powders fromlow-molecular-weight PTFE particles by granulation treatment, and thegranulation treatment is carried out at a temperature of not lower than80° C. but lower than 100° C. using a low-molecular-weightpolytetrafluoroethylene aqueous dispersion (hereinafter referred to as“low-molecular-weight PTFE aqueous dispersion (a)”) resulting fromdispersion of the low-molecular-weight PTFE particles in an aqueousdispersion medium in the presence of a surfactant.

The method (1) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention can readily produce thelow-molecular-weight PTFE-based granulated powder (P) of the inventionand the low-molecular-weight PTFE-based granulated powder (Q) of theinvention among the low-molecular-weight PTFE-based granulated powdersdescribed hereinabove.

The method of granulation treatment in the method (1) of producinglow-molecular-weight PTFE-based granulated powders is sometimes referredto herein as “emulsification/warm or hot water granulation”.

The aqueous dispersion medium in the low-molecular-weight PTFE aqueousdispersion (a) is water or a solution of a water-soluble organic solventin water and serves as a dispersion medium capable of dispersing thelow-molecular-weight PTFE particles in the presence of a surfactant. Theaqueous dispersion medium may contain an additive or additives generallyused in granulation methods using water. In cases where thelow-molecular-weight PTFE particles constitute a dispersoid dispersed inthe dispersion obtained by emulsion polymerization, for instance, theaqueous dispersion medium in the dispersion as such may also be used asthe aqueous dispersion medium in question.

The above-mentioned low-molecular-weight PTFE aqueous dispersion (a),which is a dispersion comprising the low-molecular-weight PTFE particlesdispersed in an aqueous dispersion medium in the presence of asurfactant, may further contain a water-insoluble liquid.

The low-molecular-weight PTFE aqueous dispersion (a) either contains awater-insoluble liquid or does not contain any water-insoluble liquid,and the content of the water-insoluble liquid is preferably not higherthan 5% by mass relative to the low-molecular-weight PTFE aqueousdispersion. When it is higher than 5% by mass, it tends to becomedifficult to produce the low-molecular-weight PTFE-based granulatedpowder (P) of the invention or the low-molecular-weight PTFE-basedgranulated powder (Q) of the invention. Preferably, thelow-molecular-weight PTFE aqueous dispersion is substantially free ofany water-insoluble liquid, more preferably quite free of anywater-insoluble liquid.

The low-molecular-weight PTFE aqueous dispersion (a) may contain awater-insoluble liquid if the water-insoluble liquid can swell thelow-molecular-weight PTFE particles but preferably is one free of anywater-insoluble liquid.

The water-insoluble liquid is not particularly restricted but may be anyof those which occur as liquids at ordinary temperature, namely at about30° C. and are insoluble in water. As examples thereof which enable theproduction of the low-molecular-weight PTFE-based granulated powder (P)of the invention or the low-molecular-weight PTFE-based granulatedpowder (Q) of the invention within the above content range, there may bementioned, among others, hydrocarbons such as n-hexane, cyclohexane andheptane; halogenated hydrocarbons such as dichloromethane,dichloroethane and chloroform; nitrogen-containing liquids such asN-methylpyrrolidone; esters such as ethyl acetate; and carbonate esterssuch as diethylene carbonate. Preferred as the water-insoluble liquidare halogenated hydrocarbons and, among them, dichloromethane is morepreferred.

The surfactant to be used in carrying out the method (1) of producinglow-molecular-weight PTFE-based granulated powders according to theinvention is not particularly restricted but preferably is a nonionicsurfactant, more preferably a polyoxyethylenepolyoxypropylene glycol[PPG] type surfactant.

The PPG type surfactant is preferably one having an average molecularweight of 1000 to 20000. The surfactant is used preferably in an amountof not smaller than 0.001% by mass, more preferably not larger than 0.5%by mass, still more preferably not larger than 0.1% by mass but withinthe above range.

According to the method (1) of producing low-molecular-weight PTFE-basedgranulated powders according to the invention, the granulation iscarried out while stirring the low-molecular-weight PTFE aqueousdispersion (a), and the rate of stirring and other conditions can beappropriately selected. In carrying out the method (1) of producinglow-molecular-weight PTFE-based granulated powders according to theinvention, the temperature of the low-molecular-weight PTFE aqueousdispersion (a) is preferably lower than 100° C. when thelow-molecular-weight PTFE aqueous dispersion (a) does not contain anywater-insoluble liquid. A more preferred lower limit is 85° C.

The method (2) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention is a method of producinglow-molecular-weight PTFE-based granulated powders which comprisesproducing the low-molecular-weight PTFE-based granulated powders fromlow-molecular-weight PTFE particles by granulation treatment, and thegranulation treatment is carried out using an low-molecular-weightpolytetrafluoroethylene aqueous dispersion (hereinafter referred to as“low-molecular-weight PTFE aqueous dispersion (b)”) resulting fromdispersion of the low-molecular-weight PTFE particles in an aqueousdispersion medium in the presence or absence of a surfactant and thelow-molecular-weight PTFE aqueous dispersion (b) contains awater-insoluble liquid. The content of the surfactant is not higher than5 parts by mass per 100 parts by mass of the low-molecular-weight PTFEparticles.

The method (2) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention can readily produce thelow-molecular-weight PTFE-based granulated powder (P) of the inventionand the low-molecular-weight PTFE-based granulated powder (R) of theinvention among the low-molecular-weight PTFE-based granulated powdersdescribed hereinabove.

The aqueous dispersion medium in the low-molecular-weight PTFE aqueousdispersion (b) is water or a solution of a water-soluble organic solventin water and serves as a dispersion medium capable of dispersing thelow-molecular-weight PTFE particles in the presence or absence of asurfactant. The aqueous dispersion medium may contain an additive oradditives generally used in granulation methods using water. The aqueousdispersion medium may be the same one as described hereinabove referringto the low-molecular-weight PTFE aqueous dispersion (a).

The low-molecular-weight PTFE aqueous dispersion (b) contains awater-insoluble liquid.

The content of the water-insoluble liquid is preferably not lower than5% by mass of the low-molecular-weight PTFE aqueous dispersion (b). Whenit is lower than 5% by mass, it tends to become difficult to produce thelow-molecular-weight PTFE-based granulated powder (P) of the inventionor the low-molecular-weight PTFE-based granulated powder (R) of theinvention.

As the water-insoluble liquid, there may be mentioned the same ones asdescribed hereinabove referring to the low-molecular-weight PTFE aqueousdispersion (a).

As the granulation treatment in the case of the low-molecular-weightPTFE aqueous dispersion (b) containing no surfactant, there may bementioned the underwater granulation method described hereinabove and,as the granulation treatment in the case of the low-molecular-weightPTFE aqueous dispersion (b) containing a surfactant, there may bementioned the emulsification/dispersion granulation method describedhereinabove.

The surfactant to be used in carrying out the method (2) of producinglow-molecular-weight PTFE-based granulated powders according to theinvention is not particularly restricted but includes, for example,those given hereinabove referring to the low-molecular-weight PTFEaqueous dispersion (a).

When the emulsification/dispersion granulation method, for instance, iscarried out in the presence of a surfactant, the surfactant is used inan amount of 0.001 to 0.5 part by weight per 100 parts by weight of thelow-molecular-weight PTFE particles. A more preferred lower limit is0.005 part by mass, and a more preferred upper limit is 0.1 part bymass.

In carrying out the method (2) of producing low-molecular-weightPTFE-based granulated powders according to the invention, thewater-insoluble liquid and surfactant are as described hereinabove, andthe proportion of the low-molecular-weight PTFE particles in thelow-molecular-weight PTFE aqueous dispersion (b) and other conditions,except for the granulation temperature, are the same as described abovereferring to the method (1) of producing low-molecular-weight PTFE-basedgranulated powders according to the invention and can be appropriatelyselected.

In carrying out the method (2) of producing low-molecular-weightPTFE-based granulated powders according to the invention, thelow-molecular-weight PTFE aqueous dispersion (b) may not be heated andthe temperature is preferably not higher than T° C. The term “T° C.” asused herein means the temperature at which the rate of evaporation ofthe water-insoluble liquid heated suddenly increases.

The low-molecular-weight PTFE-based granulated powders obtained by themethod (2) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention have a relatively large average graindiameter and a small angle of repose and, therefore, are superior inthat they are hardly blown up and, in addition, their handleability isgood.

The low-molecular-weight PTFE-based granulated powders obtained by themethod (2) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention will not allow the particles thereofto be blown up and, therefore, other products occurring in such anenvironment that the low-molecular-weight PTFE-based granulated powdersare handled can be prevented from being adulterated or contaminated withthe powders. The adhesion of the powders to hoppers is slight and thepowders are excellent in handleability. At the same time, thelow-molecular-weight PTFE-based granulated powders obtained by themethod (2) of producing low-molecular-weight PTFE-based granulatedpowders according to the invention can be prevented from being blown up,making it possible to preserve a good working environment.

Even if each grain of the granulated powder is formed as a result ofaggregation of a plurality of particles, which are elementary unitsconstituting the granulated powder, the low-molecular-weight PTFE-basedgranulated powder of the invention is readily disintegrated into theparticles before granulation on the occasion of admixing, as an additiveto base materials, with such other base materials as engineeringplastics in various mixers, hence can be said to be excellent indispersibility in and miscibility with the base materials. Thelow-molecular-weight PTFE-based granulated powder of the invention showsequally excellent dispersibility in and miscibility also on the occasionof feeding to a single-screw extruder together with polybutyleneterephthalate [PBT] and glass fiber, as disclosed in Japanese KokaiPublication S59-140253, or in the case of addition on the occasion offeeding to an extruder, as disclosed in Japanese Kokai PublicationS60-223852.

Those molding powder-based granulation products which are obtained bysubjecting molding powders of polytetrafluoroethylene species having anumber average molecular weight exceeding 600000 to the same granulationtreatment as the granulation treatment for obtaining thelow-molecular-weight PTFE-based granulated powder of the inventionhardly disintegrate into the particles before granulation even in thestep of admixing with resins or other materials.

The low-molecular-weight PTFE powder (B) of the invention has a meltviscosity of not higher than 2500 Pa·s as measured by making ameasurement at 340° C. by the flow tester method.

The low-molecular-weight PTFE powder (B) of the invention can be used asthe low-molecular-weight PTFE powder comprising low-molecular-weightPTFE particles in the low-molecular-weight PTFE-based granulated powderof the invention and further can be used as the low-molecular-weightPTFE powder comprising low-molecular-weight PTFE particles in the methodof producing low-molecular-weight PTFE-based granulated powdersaccording to the invention, as described hereinabove, hence, as isevident from the facts mentioned above, it is included among thoselow-molecular-weight PTFE powders. Among the above-mentionedlow-molecular-weight PTFE powders, it has a melt viscosity within therange given above. Preferably, the melt viscosity of thelow-molecular-weight PTFE powder (B) of the invention is not higher than2000 Pa·s.

The low-molecular-weight PTFE powder (B) of the invention can be readilyobtained by suspension polymerization. The method of producinglow-molecular-weight PTFE powders according to the invention, whichmethod is to be described later herein, is preferably used as thesuspension polymerization for obtaining the low-molecular-weight PTFEpowder (B) of the invention.

The low-molecular-weight PTFE powder (B) of the invention has a meltviscosity within the above range and, in addition, preferably has aspecific surface area of smaller than 7 m²/g. A more preferred upperlimit to the low-molecular-weight PTFE powder (B) of the invention is 6m²/g, a still more preferred upper limit is 5 m²/g, and a more preferredlower limit is 1 m²/g and a still more preferred lower limit is 2 m²/g.

The low-molecular-weight PTFE powder (B) of the invention is preferablyone having a melt viscosity within the above range and a specificsurface area within the same range as given for the low-molecular-weightPTFE powder (A) as obtained by suspension polymerization.Low-molecular-weight PTFE powders having a melt viscosity within theabove range and a specific surface area within the above range can beeasily obtained by producing the same using the suspensionpolymerization technique.

The method of producing low-molecular-weight polytetrafluoroethylenepowders according to the invention (hereinafter referred to as “methodof producing low-molecular-weight PTFE powders”) comprises producing theabove-mentioned low-molecular-weight PTFE powders by suspensionpolymerization using a chain transfer agent.

The method of producing low-molecular-weight PTFE powders according tothe invention employs suspension polymerization and is superior in thatcoagulation is not required as compared with the case of production byemulsion polymerization. The method of producing low-molecular-weightPTFE powders according to the invention is suited for use not only inproducing the low-molecular-weight PTFE powder (B) but also in producingthe low-molecular-weight PTFE powder (A) mentioned hereinabove.

The chain transfer agent is the same one as described hereinabovereferring to the production of low-molecular-weightpolytetrafluoroethylene by polymerization mentioned hereinabove.

The chain transfer agent is used preferably in an amount of 0.01 to 0.5mole percent relative to the gaseous phase at the start ofpolymerization.

When the suspension polymerization is carried out at a liquidtemperature of not lower than 40° C. but lower than 100° C., thepolymerization initiator to be used includes persulfate salts or sulfitesalts, and organic peroxides.

When, assuming that the “persulfate salts and sulfite salts” arereferred to as “group a” and the “organic peroxides” as “group b”, atleast one member is selected from each of group a and group b for use asthe polymerization initiator, another reagent having polymerizationinitiating activity may also be used. The persulfate and sulfite saltshave a short half-life and act as polymerization initiators from thebeginning of polymerization whereas the organic peroxides are relativelylong in half-life and begin to act as polymerization initiators moreslowly than the persulfate salts and sulfite salts. Therefore, thecombined use of both can render the molecular weight distributionnarrower and sharper.

The persulfate salts are not particularly restricted but include, amongothers, ammonium persulfate and potassium persulfate. The sulfite saltsare not particularly restricted, either, but include, among others,ammonium sulfite and potassium sulfite. The organic peroxides are notparticularly restricted but include, among others, benzoyl peroxide,disuccinoyl peroxide and diglutaroyl peroxide.

Besides the above-mentioned polymerization initiators, a redox catalyst,which is to be mentioned later herein, may also be used.

The “liquid temperature” mentioned above is the temperature of thepolymerization reaction mixture in liquid form.

When the suspension polymerization is carried out at a liquidtemperature of 5 to 40° C., the polymerization initiator to be usedcomprises not only a persulfate salt or sulfite salt and/or an organicperoxide but also a redox catalyst. When it comprises a redox catalyst,the reaction can be allowed to proceed even at such a low temperature of5 to 40° C.

The above-mentioned combination “not only a persulfate salt or sulfitesalt and/or an organic peroxide but also a redox catalyst” may be any ofthe five combinations: persulfate salt and redox catalyst, sulfite saltand redox catalyst, organic peroxide and redox catalyst, persulfate saltand organic peroxide and redox catalyst, and sulfite salt and organicperoxide and redox catalyst.

The persulfate salt, sulfite salt, organic peroxide and redox catalysteach may comprise two or more species.

Useful as the persulfate salt, sulfite salt and organic peroxide arethose mentioned hereinabove.

The redox catalyst is not particularly restricted but includes, forexample, a metal carbonyl-carbon tetrachloride mixture and aperoxide-iron(II) compound mixture, among others.

The low-molecular-weight polytetrafluoroethylene-based gelation productpowder (hereinafter referred to as “low-molecular-weight PTFE gelationproduct powder”) of the invention is one obtained upon heat treatment ofthe above-mentioned low-molecular-weight PTFE powder at a temperaturenot lower than 250° C. but lower than 340° C.

In the above heat treatment, a preferred lower limit to the temperatureis 300° C., and a preferred upper limit to the temperature is themelting point of the low-molecular-weight PTFE, for example 330° C.

The low-molecular-weight PTFE gelation product powder andlow-molecular-weight PTFE-based granulated powder gelation product eachmay be either in a “complete gelation product” state after completegelation of all particles of the low-molecular-weight PTFE powder or allparticles of the low-molecular-weight PTFE-based granulated powder, orin a “semi-gelation product” state after gelation of part of particlesand/or partial gelation of particles.

The above heat treatment results in mutual fusion bonding, through pointcontacting, of particles/grains of the low-molecular-weight PTFE powderor low-molecular-weight PTFE-based granulated powder to form massesresulting from binding up generally by weak bonding strength.

The above heat treatment also results in increases in momentum ofpolymer chains in individual particles/grains of thelow-molecular-weight PTFE powder or low-molecular-weight PTFE-basedgranulated powder, hence in mutual entanglement of the polymer chains;as a result, there is a tendency for the respective particles/grains tobecome smaller in size and take a compact structure, namely to shrink.The respective grains obtained by the above heat treatment are generallyhigher in apparent density than the low-molecular-weight PTFE powder orlow-molecular-weight PTFE-based granulated powder, which is the powderbefore heat treatment, and therefore are more resistant to being blownup and more improved in powder flowability, hence in their behavior infeeding to hoppers.

The lumpy product obtained by the above heat treatment may be subjectedto grinding treatment to attain a desired size. The grinding treatmentis preferably carried out in a manner such that the product may bedisintegrated into the respective grains of the low-molecular-weightPTFE gelation product powder or low-molecular-weight PTFE-basedgranulated powder-derived gelation product powder.

An additive can also be prepared which comprises thelow-molecular-weight PTFE-based granulated powder, low-molecular-weightPTFE-based granulated powder gelation product, low-molecular-weight PTFEpowder or low-molecular-weight PTFE gelation product powder.

The additive may be the low-molecular-weight PTFE-based granulatedpowder itself, the low-molecular-weight PTFE-based granulated powdergelation product itself, the low-molecular-weight PTFE powder itself orthe low-molecular-weight PTFE gelation product powder itself, or may bea composite additive with a wax or the like added thereto. The compositeadditive supplemented with a wax is used in the field of inks, forinstance.

The above additive is incorporated in various other base materialsaccording to the intended purpose. As the other base materials, theremay be mentioned engineering plastics such as polyoxybenzoyl polyesters,polyimides, polyamides, polyamideimides, polyacetals, polycarbonates andpolyphenylene sulfide, and other molding materials; inks; and coatingmaterials, among others.

The use of the above additive is not particularly restricted but theadditive may be used, for example, for the purpose of improving thenonstickiness and sliding characteristics of copying rolls; andimproving the slip characteristics of inks, varnishes, paints and othercoating materials as well as cosmetics such as foundations. Further, itis suited for use in those fields in which engineering plasticsmoldings, such as furniture surface layer sheets, automobile dashboardsand domestic electric appliances coverings, should be improved intexture, or machine parts generating mechanical friction, for examplelight load bearings, gears, cams, touch-tone phone buttons, projectors,camera parts and sliding members, should be improved in slipcharacteristics and/or wear resistance, or waxes and the like should beimproved in oil repellency or water repellency, for instance, or for useas processing aids for engineering plastics.

The above-mentioned low-molecular-weight PTFE-based granulated powder,low-molecular-weight PTFE-based granulated powder gelation product,low-molecular-weight PTFE powder, or low-molecular-weight PTFE gelationproduct powder can also be used as a molding material for obtainingmolded articles. The molded articles may contain a filler and/or an oil.The filler is not particularly restricted but includes, among others,engineering plastics such as polyoxybenzoyl polyesters, polyimides,polyamides, polyamideimides, polyacetals, polycarbonates andpolyphenylene sulfide; carbon fibers; glass fibers; bronze powders;graphite powders; calcium carbonate; calcium sulfate; molybdenumdisulfide; silicate minerals such as chlorite, talc and mica; metaloxides; fine powders of soft metals, etc.

BEST MODES FOR CARRYING OUT THE INVENTION

The following examples illustrate the present invention in furtherdetail. These examples are, however, by no means limitative of the scopeof the invention.

[Low-molecular-weight PTFE Powder Polymerization]

POLYMERIZATION EXAMPLE 1

A 150-liter stainless steel polymerization vessel equipped with a conetype agitating element was charged with 70 L of deionized water and thentightly closed. After evacuation of the vessel, 100 g of ethane wascharged. The TFE monomer was then charged until the vessel insidepressure arrived at 0.5 MPa, and the whole charge was heated to 85° C.After arrival of the vessel inside temperature at 85° C., the TFEmonomer was again added to adjust the pressure to 0.8 MPa. Upon additionof 250 ppm/H₂O of ammonium persulfate and 250 ppm/H₂O of disuccinoylperoxide each as a polymerization initiator in an aqueous solution form,the consumption of the TFE monomer in the vessel started immediately.During polymerization, each time the consumption resulted in a drop invessel inside pressure to 0.7 MPa, TFE was additionally fed to raise thepressure to 0.85 MPa. After repetitions of this procedure, when the feedof TFE amounted to 12 kg, the polymerization reaction was discontinued,and the vessel inside pressure was released to ordinary pressure. Afterthe start of the polymerization reaction, the vessel inside temperaturewas always maintained at 84.5 to 85.5° C. After cooling the vesselinside to room temperature, the particles obtained were washed withdeionized water, then filtered off and dried at 170° C. in a hot aircirculation type drier for 12 hours to give a low-molecular-weight PTFEpowder.

POLYMERIZATION EXAMPLE 2

A low-molecular-weight PTFE powder was obtained in the same manner as inPolymerization Example 1 except that the amount of ethane charged was140 g.

POLYMERIZATION EXAMPLE 3

A low-molecular-weight PTFE powder was obtained in the same manner as inPolymerization Example 1 except that the amount of ethane charged was160 g.

POLYMERIZATION EXAMPLE 4

A low-molecular-weight PTFE powder was obtained in the same manner as inPolymerization Example 1 except that the amount of ethane charged was200 g.

POLYMERIZATION EXAMPLE 5

A low-molecular-weight PTFE powder was obtained in the same manner as inPolymerization Example 1 except that the amount of ethane charged was 60g.

POLYMERIZATION EXAMPLE 6

A low-molecular-weight PTFE powder was obtained in the same manner as inPolymerization Example 1 except that the amount of ethane charged was 75g.

The low-molecular-weight PTFE powders obtained were evaluated for thefollowing physical characteristics.

Apparent Density

Measurements were made according to JIS K 6891-5.3.

Average Particle Size

Using a laser diffraction type particle size distribution analyzer(product of Nippon Denshi (JEOL)) and using no cascade, particle sizedistributions were measured at a pressure of 0.1 MPa for a measurementtime of 3 seconds. For each powder, the average particle size wasregarded as equal to the particle size corresponding to the 50% level ofthe cumulative particle size distribution.

High Temperature Volatility

Each sample (10 g; A grams in the formula given below) was placed in analuminum cup (capacity 50 ml, upper diameter 61 mm, lower diameter 42mm, depth 33 mm), the whole was maintained at 300±2° C. in a nitrogenatmosphere in a hot air circulation type electric oven adjusted inadvance to the heating temperature for 1 hour and, thereafter, the massof the sample was measured. The high temperature volatility wascalculated according to the formula:High temperature volatility (% by mass)=[{A−mass after heat treatment(g)}×100]/AMelt Viscosity

A 2-g portion of each sample heated beforehand at a temperature of 340°C. for 5 minutes was subjected to measurement at 340° C. under a load of0.7 MPa using a flow tester (product of Shimadzu Corp.) with a 2ø-8Ldie.

Specific Surface Area

Measurements were made by the BET method using a surface analyzer(trademark: Monosorb, product of Quantachrome). The carrier gas used wasa mixed gas composed of 30% of nitrogen and 70% of helium, and liquidnitrogen was used for cooling.

The results of the above tests are shown in Table 1.

TABLE 1 Chain transfer agent Apparent Average High temperature Meltviscosity Specific (ethane) density particle size volatility (Pa · s)surface area Addition level (g) (g/cm³) (μm) (%) (340° C.) (m²/g)Polymerization 100 0.35 12.4 0.223 1292 3.8 Example 1 Polymerization 1400.28 4.2 0.456 790 3.5 Example 2 Polymerization 160 0.30 4.2 0.421 2423.5 Example 3 Polymerization 200 0.29 3.8 0.309 80 2.8 Example 4Polymerization 60 0.44 15.4 0.333 2424 4.2 Example 5 Polymerization 750.39 14.8 0.284 1768 4.2 Example 6

From Table 1, it was revealed that an increased level of addition of thechain transfer agent ethane results in a reduced melt viscosity.

[Granulation Treatment of Low-Molecular-Weight PTFE Powders]

EXAMPLE 1 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Underwater Granulation Method

A 15-L stirring vessel equipped with a cone blade was charged with 6.0kg of deionized water, and the temperature was adjusted in advance to 20to 22° C. (step I). Then, the stirring vessel was charged with 500 g ofa low-molecular-weight PTFE powder (apparent density 0.28 g/cm³, averageparticle size 3.1 μm, high temperature volatility 0.21% by mass, angleof repose 42.1 degrees), and stirring was effected by rotating the coneblade at 800 to 900 rpm (step II). After 1 minute of stirring, 200 ml ofdichloromethane was added slowly (step III). After the subsequent 10minutes of stirring, the liquid and the solid matter were separated fromeach other using a 200-mesh sieve (step IV). The solid matter filteredoff was dried in a hot air drying oven at 170° C. for 24 hours and thencooled to room temperature to give a low-molecular-weight PTFE-basedgranulated powder (step V).

The low-molecular-weight PTFE-based granulated powder obtained wastested for the above-mentioned apparent density and high temperaturevolatility and evaluated for the following physical characteristics:

Average Grain Diameter

The average grain diameter (100 μm or larger) of a powder was measuredin accordance with ASTM D 4894 by the same method as described inInternational Publication WO 99/12996, page 12, line 6 from bottom topage 13, line 7 from top.

The average diameter smaller than 100 μm of a powder was measuredaccording to the method described above for the polymerization examples.

Angle of Repose

Measurements were made using a powder tester (trademark: PN-R, productof Hosokawa Micron Corp.) according to the angle of repose measurementmethod specified for that tester.

EXAMPLES 2 TO 5 Production of Low-Molecular-Weight PTFE-Based GranulatedPowders by the Underwater Granulation Method

Low-molecular-weight PTFE-based granulated powders were obtained in thesame manner as in Example 1 except that the amount of dichloromethanecharged in step III was 300, 400, 450 or 500 ml, respectively.

EXAMPLE 6 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Underwater Granulation Method

A low-molecular-weight PTFE-based granulated powder was obtained in thesame manner as in Example 3 except that the stirring in step IV inExample 1 was carried out for 30 minutes.

EXAMPLE 7 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Underwater Granulation Method

A low-molecular-weight PTFE-based granulated powder was obtained in thesame manner as in Example 3 except that, in step IV in Example 1, thetemperature of the stirring vessel was raised to 33° C. over 15 minutesand then the stirring was carried out for 25 minutes.

COMPARATIVE EXAMPLE 1 Production of a Low-Molecular-Weight PTFE-BasedGranulated Powder by the Underwater Granulation Method

A low-molecular-weight PTFE-based granulated powder was obtained in thesame manner as in Example 3 except that, in step IV in Example 1, thetemperature of the stirring vessel was raised to 36° C. over 15 minutesand then the stirring was carried out for 10 minutes.

EXAMPLE 8 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Emulsification/Dispersion Granulation Method

A low-molecular-weight PTFE-based granulated powder was obtained in thesame manner as in Example 3 except that, in step III in Example 1, aspecific surfactant was added in an amount of 0.025% by mass relative tothe low-molecular-weight PTFE prior to the charging of 400 ml ofdichloromethane. Here and hereinafter, the “specific surfactant” meanspolyoxyethylenepolyoxypropylene glycol type nonionic surfactant(trademark: Pronon #104, Product of Nippon Yushi (NOFCO); averagemolecular weight 1670).

EXAMPLE 9 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Emulsification/Dispersion Granulation Method

A low-molecular-weight PTFE-based granulated powder was obtained in thesame manner as in Example 3 except that the above specific surfactantwas added in an amount of 0.010% by mass relative to thelow-molecular-weight PTFE.

EXAMPLE 10 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Emulsification/Dispersion Granulation Method

A low-molecular-weight PTFE-based granulated powder was obtained in thesame manner as in Example 3 except that the above specific surfactantwas added in an amount of 0.005% by mass relative to thelow-molecular-weight PTFE.

EXAMPLE 11 Production of a Low-Molecular-Weight PTFE-Based GranulatedPowder by the Emulsification/Hot Water Granulation Method

A 15-L stirring vessel equipped with a cone blade was charged with 6.0kg of deionized water, and the temperature was adjusted in advance to 20to 22° C. (step 1). Then, the stirring vessel was charged with 500 g ofthe same low-molecular-weight PTFE as used in Example 1, and stirringwas effected by rotating the cone blade at 1000 rpm (step 2). After 1minute of stirring, the specific surfactant mentioned above was added inan amount of 0.025% by mass relative to the low-molecular-weight PTFE(step 3). The vessel inside temperature was raised to 95° C. withstirring (step 4). After the subsequent 10 minutes of stirring at 95°C., the liquid and the solid matter were separated from each other usinga 200-mesh sieve (step 5). The solid matter filtered off was dried in ahot air drying oven at 170° C. for 24 hours (step 6). The dried solidmatter was cooled to room temperature to give a low-molecular-weightPTFE-based granulated powder (step 7).

EXAMPLE 12 TO 14 Production of Low-Molecular-Weight PTFE-BasedGranulated Powders by the Emulsification/Hot Water Granulation Method

Low-molecular-weight PTFE-based granulated powders were obtained in thesame manner as in Example 11 except that, in step 3, the specificsurfactant was charged in the respective amounts specified in Table 2.

COMPARATIVE EXAMPLES 1 TO 6

Commercial low-molecular-weight PTFE powders (ungranulated ones) weresubjected to physical characteristics evaluation in the same manner asin Example 1.

The results obtained in the above manner are shown in Table 2. In thetable, #104 denotes the above-mentioned polyoxyethylenepolyoxypropyleneglycol type nonionic surfactant (Pronon #104).

TABLE 2 Average High Granulation conditions Apparent particle Angle oftemperature Surfactant Water-insoluble liquid Temper- Stirring densitysize repose volatility Granulation Mass %/ Addition ature period (g/cm³)(μm) (degrees) (%) method Species polymer Species level (ml) raising(min.) Starting 0.280 3.1 42.1 0.21 — — — — — — — PTFE Example 1 0.401185 48.0 0.23 Underwater — — CH₂Cl₂ 200 None 10 Example 2 0.570 247 37.60.21 granulation — — CH₂Cl₂ 300 None 10 Example 3 0.620 530 34.2 0.24 —— CH₂Cl₂ 400 None 10 Example 4 0.600 664 35.3 0.28 — — CH₂Cl₂ 450 None10 Example 5 0.600 912 34.8 0.30 — — CH₂Cl₂ 500 None 10 Example 6 0.684761 38.1 0.22 — — CH₂Cl₂ 400 None 30 Example 7 0.639 569 38.6 0.24 — —CH₂Cl₂ 400 33° C. 25 Compar. 0.304 5 44.1 0.30 Underwater — — CH₂Cl₂ 40036° C. 10 Granulation granulation Ex. 1 Example 8 0.594 357 37.5 0.44Emulsification/ #104 0.025 CH₂Cl₂ 400 None 10 Example 9 0.590 446 36.00.40 dispersion 0.010 CH₂Cl₂ 400 None 10 Example 10 0.607 442 35.0 0.39granulation 0.005 CH₂Cl₂ 400 None 10 Example 11 0.324 4.2 49.0 0.14Emulsification/ #104 0.025 — — 95° C. 10 Example 12 0.328 3.6 53.4 0.29hot water 2.5 — — 95° C. 10 Example 13 0.530 4.2 50.1 0.12 granulation0.250 — — 95° C. 10 Example 14 0.390 3.8 49.6 0.11 0.100 — — 95° C. 10Compar. 0.340 3.3 46.2 0.12 Not granulated Ex. 1 Compar. 0.380 5.0 47.40.07 Ex. 2 Compar. 0.400 4.7 46.5 0.08 Ex. 3 Compar. 0.490 7.4 42.8 0.03Ex. 4 Compar. 0.460 7.6 54.3 0.34 Ex. 5 Compar. 0.420 12.4 — 0.28 Ex. 6

From Table 2, it was revealed that the low-molecular-weight PTFE-basedgranulated powders obtained in Examples 11 to 14 by theemulsification/hot water granulation method were 1.15 to 1.35 timesgreater in average grain size, 1.15 to 1.9 times higher in apparentdensity and 1.1 to 1.3 times greater in angle of repose as compared withthe starting PTFE.

[Gelation of a Low-Molecular-Weight PTFE Powder]

GELATION EXAMPLE 1

The dried low-molecular-weight PTFE powder obtained in PolymerizationExample 2 was spread on a stainless steel tray so that the thicknessmight not exceed 20 mm. The tray was placed in a hot air circulationtype electric oven heated in advance to 250° C. and the powder was heattreated for 30 minutes. After the lapse of 30 minutes, the tray wasimmediately taken out and allowed to cool to room temperature. Alow-molecular-weight PTFE gelation product powder was thus obtained.

GELATION EXAMPLE 2

A low-molecular-weight PTFE gelation product powder was obtained in thesame manner as in Gelation Example 1 except that the temperature in thehot air circulation type electric oven was 300° C.

GELATION EXAMPLE 3

A low-molecular-weight PTFE gelation product powder was obtained in thesame manner as in Gelation Example 1 except that the temperature in thehot air circulation type electric oven was 320° C.

GELATION EXAMPLE 4

A low-molecular-weight PTFE gelation product powder was obtained in thesame manner as in Gelation Example 1 except that the temperature in thehot air circulation type electric oven was 330° C.

COMPARATIVE GELATION EXAMPLE 1

A low-molecular-weight PTFE gelation product powder was obtained in thesame manner as in Gelation Example 1 except that the temperature in thehot air circulation type electric oven was 340° C.

COMPARATIVE GELATION EXAMPLE 2

A low-molecular-weight PTFE gelation product powder was obtained in thesame manner as in Gelation Example 1 except that the temperature in thehot air circulation type electric oven was 200° C.

The low-molecular-weight PTFE gelation product powders obtained weresubjected to the above-mentioned apparent density, average graindiameter and melt viscosity measurements and further to the followingphysical characteristics measurements.

Heat of Fusion

Melting peak area measurements were carried out using a differentialscanning calorimeter (trademark: DSC-50, product of Shimadzu Corp.).

Sensory Testing of Powders for the Tendency Toward Being Blown Up

A macroscopic evaluation was made according to the following criteria:

-   ⊚—No blowing up observed.-   ◯—Almost no blowing up observed.-   Δ—A slight extent of blowing up observed.-   X—Large amount blowing up observed.

The results obtained in the above manner are shown in Table 3.

TABLE 3 Heating Apparent Average Melt viscosity Heat of Sensory testingtemperature density particle size (Pa · s) fusion for powder (° C.)(g/cm³) (μm) (340° C.) (mJ/g) blowing up Gelation Example 1 250 0.35 4.2690 71 ◯ Gelation Example 2 300 0.41 5.0 670 74 ⊚ Gelation Example 3 3200.42 5.4 710 76 ⊚ Gelation Example 4 330 0.55 8.2 694 68 ⊚ Comparative340 Fusion bonding of particles Gelation Example 1 Comparative 200 0.303.6 790 73 x Gelation Example 2

From Table 3, it was revealed that, in the gelation examples, a highergelation temperature results in a higher apparent density and a greateraverage grain diameter, hence in a greater extent of inhibition ofblowing up.

INDUSTRIAL APPLICABILITY

The method of producing low-molecular-weight PTFE powders according tothe invention, which has the constitution described above, can givelow-molecular-weight PTFE powders reduced in tendency toward being blownup and improved in handleability.

1. A low-molecular-weight polytetrafluoroethylene-based granulatedpowder which is obtained by a granulation treatment of alow-molecular-weight polytetrafluoroethylene particle consistingessentially of a low-molecular-weight polytetrafluoroethylene having anumber average molecular weight of not higher than 600000, whichgranulated powder has an apparent density of not less than 0.401 g/cm³and an average grain diameter of not less than 185 μm.
 2. Thelow-molecular-weight polytetrafluoroethylene-based granulated powderaccording to claim 1, wherein the average grain diameter of said lowmolecular-weight polytetrafluoroethylene-based granulated powder is 1 to400 times the average particle diameter of a low-molecular-weightpolytetrafluoroethylene powder comprising the low-molecular-weightpolytetrafluoroethylene particle and the apparent density of said lowmolecular-weight polytetrafluoroethylene-based granulated powder is 1.15to 4 times the apparent density of said low-molecular-weightpolytetrafluoroethylene powder comprising said low-molecular-weightpolytetrafluoroethylene particle.
 3. The low-molecular-weightpolytetrafluoroethylene-based granulated powder according to claim 1,wherein the average grain diameter of said low molecular-weightpolytetrafluoroethylene-based granulated powder is 10 to 400 times theaverage particle diameter of a low-molecular-weightpolytetrafluoroethylene powder comprising the low-molecular-weightpolytetrafluoroethylene particle and the angle of repose of said lowmolecular-weight polytetrafluoroethylene-based granulated powder is lessthan 1 time the angle of repose of said low-molecular-weightpolytetrafluoroethylene powder comprising said low-molecular-weightpolytetrafluoroethylene particle.
 4. The low-molecular-weightpolytetrafluoroethylene-based granulated powder according to claim 1,wherein, as for a low-molecular-weight polytetrafluoroethylene powdercomprising the low-molecular-weight polytetrafluoroethylene particle,said low-molecular-weight polytetrafluoroethylene powder has: (i) aspecific surface area smaller than 7 m²/g and (ii) a melt viscosity nothigher than 2500 Pa·s as measured by the flow tester method at 340° C.